Liquid crystal (LC) microdisplays are widely found in projections systems, as for example used in business presentations and home entertainment (e.g., large screen televisions). In general, these relatively small displays (e.g., typically measuring less than 1.5″ diagonally) are coupled with one or more optical lenses that enlarge the projected image to a suitable display size. Conventionally, LC microdisplays have been based on one of two types of technologies, namely a reflective-type microdisplay (e.g., liquid crystal on silicon (LCoS)) or a transmissive-type microdisplay.
A typical transmissive LC microdisplay includes a layer of liquid crystal material (e.g., vertical-aligned (VA)-mode, in-plane switching (IPS)-mode, planar aligned (PA)-mode, or, more commonly, 90 degree twisted nematic (TN)-mode) sandwiched between front and back transparent plates. The back plate includes a patterned electrode layer, while the front plate includes a common electrode layer, each of which is typically formed from a transparent material such as indium tin oxide (ITO). When 90 degree TN-mode LC is used, the front and back plates typically include alignment layers that are aligned perpendicular to each other such that the LC molecules are arranged in a helical structure or twist. In the absence of an applied voltage (i.e., off-state), the twisted arrangement rotates the polarization of incident linearly polarized light by about 180 degrees. In the presence of a sufficiently large applied voltage (i.e., on-state), the LC molecules begin to untwist such that the polarization of the linearly polarized incident light is not rotated. This LC cell is typically disposed between a polarizer and analyzer having parallel transmission axes (i.e., normally-black) or, more commonly, perpendicular transmission axes (i.e., normally-white).
In commercially available systems, transmissive LC microdisplays typically use an active matrix system, wherein a matrix of thin-film transistors (TFTs) controls the voltage applied by the electrodes. More specifically, each TFT functions as a switching element that controls the orientation of LC in each display pixel. Conventionally, the active TFT layer has been formed by depositing silicon (e.g., amorphous, poly, or crystalline) on a glass back plate. More recently, the advantages of using sapphire as the substrate for the silicon layer have been realized. For example, sapphire exhibits optical transparency in the visible band and is a semiconductor that promotes the growth of single crystal silicon, thus reducing manufacturing complexity and costs. In addition, sapphire is desirable for TFT fabrication because it has high electron and hole mobilities, which enables high speed logic switching. Furthermore, it has higher thermal conductivity than the conventional glass substrates used in high-temperature polysilicon TFTs, thus providing efficient heat dissipation in high brightness illumination systems.
Unfortunately, the use of sapphire as a substrate for LC TFT fabrication has been found to have a negative effect on the panel contrast ratio of LC microdisplays. For example, in U.S. Pat. No. 7,480,017, Fisher et al. teach that the use of sapphire in LC microdisplays de-polarizes the light that passes through the liquid crystal material, reducing the on/off contrast ratio. Fisher et al. teach improving the contrast ratio by incorporating a wire grid or other polarizer on the active silicon back plate to correct for the depolarization of light passing through the sapphire substrate. While the internal polarizer is stated to significantly improve the contrast of a transparent microdisplay built on sapphire substrate, the improvement is achieved at the expense of lost light intensity (e.g., the internal polarizer filters out a portion of the light transmitted through the sapphire substrate having elliptical polarization).